1. Field of the Invention
This invention relates to methods and structures known in general as fiber reinforced plastics, but more specifically as "advanced fibrous composites", which are strengthened with heat and pressure, usually in a vacuum, during a curing cycle for the production of low density materials requiring exceptional strength and stiffness.
2. Background Information
Fiberglass embedded in an epoxy matrix has been used in the manufacture of aircraft since 1940, initially in structure intended to carry light loads. As early as the late 1950's, Dr. Leo Windecker conceived and built a number of general aviation aircraft known commercially as the "Eagle", using almost entirely fiberglass-epoxy. The skins were the primary structure of the aircraft and were composed of one or more plies of nonwoven unidirectional fiber tape oriented for optimum load carrying capacity The wing spars and other laminates were composed of multiple oriented plies of fiber cloth. The resin system consisted of a Bisphenol-A epoxy resin, amine curing agents, and a silane coupling agent.
The Eagle still flies today, with a remarkable record of endurance and maintenance free performance, an indicator of the future of aviation.
By early 1981 the large commercial aircraft companies had made large strides in the use of "advanced fibrous composites". Boeing, as one example, has used composites on their commercial airplanes since the early 1960's. By 1981 advanced fibrous composites were in use in the Models 767 and 757, including fibers of graphite, Kevlar and graphite/Kevlar hybrids fabricated into skins for rudders, elevators, spoilers, ailerons, gear doors, fixed trailing edges, fairings and engine cowls. As a result, there is a weight savings of 25-30 percent in a Model 747 and annual fuel savings of about $750,000.
Initially liquid resin was poured onto glass fabric in the design configuration, then spread to impregnate the fabric. The component was placed in a subsequently evacuated bag prior to curing in an autoclave, a labor intensive and expensive procedure. Major cost savings were realized after the development of pre-impregnated fibers with epoxy resin, materials called "prepregs", which have uniformity of resin content and yield uniform properties. Improvements in the prepreg materials have made possible precision curing cycles yielding uniform composites with greatly improved properties along with additional reductions in material and labor costs.
Initial honeycomb core materials at Boeing were polyester and nylon-phenolic. Later heat resistant phenolic core was used, and more recently "Nomex". A typical structural fiberglass panel has upper and lower fiberglass skins and doublers and honeycomb core. "Tedlar" is normally applied to the upper skin as a moisture barrier. Areas of high localized loading have titanium inserts, while antenna grounding and lightning strike protection is provided by a sprayed aluminum coating or bonding aluminum foil on composite surfaces.
The scope of the advance in composite technology is seen in the successful use of rotor blades of composite materials by all of the manufacturers of helicopters. The blades use spars stiffened with graphite, combined with titanium on the leading edges and hubs, in a hybrid of metal and composites of exceptional strength and reliability.
Construction of structural members of fibrous composites requires movement into more complex shapes and methods. The use of a elastomeric interior mold, which is pressurized for rigidity while curing the composite has been one approach to the formation of complex shapes such as hollow ducts for such use as ram air intakes of aircraft. Another approach to complex composite structures involves the use of meltable cores as a removable mandrel around which composites are positioned to form hollow and parallel multi-ducted channels for the interior of a radome. This enables the circulation of warm air in the channels to de-ice the exterior of the radome. Silicone rubber mandrels having a metal core such as a flexible cable that is removable to permit deformation of the rubber and displacement from the interior of a hollow composite beam have also been used to manufacture radomes. Also, a variety of processes and machines have been used to lay composite tape and fabric in complex shapes as well as to filament-wind roving.
Even though there have been impressive strides in the use of composites and in the methods and techniques to form increasingly complex structural shapes, there exists the need for manufacturing methods and structures which can effectively take advantage of the physical and mechanical properties and manufacturing flexibilities which are characteristic of advanced fibrous composites.